Conical reflector



Feb. 22, 1955 c. v. ROBINSON 2,702,859

CONICAL REFLECTOR 7 Filed Oct. 30, 1945 INVENTOR CHARLES V. ROBINSO NATTORNEY CONICAL REFLECTOR Charles V. Robinson, Newton Center, Mass,assignor, by mesne assignments, to the United States of America asrepresented by the Secretary of the Navy Application October 30, 1945,Serial No. 625,661

11 Claims. (Cl. 250--33.65)

This invention relates to directional microwave reflector type antennasand more particularly to reflectors which, when illuminated by a linesource of radiation, reflect a beam having a plane wave front. By movingthe line source normally to itself across the aperture, a scanning beamcan be produced.

T-wo special cases of reflectors having this property are well known,namely, a parabolic cylinder and a 45 right circular cone. It is theobject of this invention to provide a reflector of such shape that whenilluminated by a line source it will reflect a plane wave frontradiation in a direction at any desired predetermined angle with areference line from the line source to the reflector. It is anotherobject to provide a reflector adaptable for producing a scanningradiation beam when it is illuminated by a line source which is causedin effect to reciprocate back and forth normal to itself across theaperture. Other objects and features will be found in the followingdescription and figures.

Fig. 1 shows one such reflector surface, being a cone whose sections byone set of parallel planes are ellipses; and

Fig. 2 shows another type of reflector composed of small plates or zonesfrom a family of elliptical cones each similar respectively to Fig. 1.

Referring to Fig. 1, let a line source 2 radiate radially to itself acylindrical wave front to feed a reflector 3 assumed to be of such shapethat a plane wave comes off from it, with the beam direction line 4making an angle a with the incident reference line 5. In order for thisto occur the reflector 3 must be a cone with apex 6 and having sectionsnormal to the line 26 which are ellipses such as 7 and 8. These ellipsesmust have an eccentricity equel to cos a and must each have one focus 11on the line 2'6. Hence the line 26 may be referred to as the focal axis.The conic element between the ellipse vertex 9 and the apex 6 shouldmake an angle of a/ 2 with the focal axis 26. For reference purposesattention is directed to the line 10 which is a latus rectum of theellipse 7 since it passes through a focus 11 and is normal to the majoraxis 5. Two special cases have been familiar heretofore. For a=0, thesurface becomes a parabolic cylinder, while for a=90 it becomes a rightcircular cone of 45 generating angle.

A family of these cones whose focal distance, such as 2-9, differ byN)\/ (1+cos a), where N is any integer and x is the wave length ofenergy in free space, will have reflected wave fronts with the samephase. A zone plate reflector for a scan antenna illuminated by amovable line source can also be constructed in the form of a number ofsmall plates or zones from such a family of elliptical cones as shown inFig. 2 so as to approximate a circular cone of generating angle a/Z andwith axis 26. The beam can now be tilted by moving the line sourcenormally to itself across the aperture. In general the wave front willbe twisted by an amount which increases with both displacement and withangle a. However, with large fanning, that is, when the beam extendsthrough a much greater angle in one plane than it does in a second planeperpendicular to the first plane, angle a may be small and the twistwill not be serious. The scanning plane will be the plane which isparallel to the latus rectum 10 and which includes the line 4, thecenter position of the beam.

Fig. 2 shows a rapid scanning antenna constructed according to theseprinciples. A moving line source is gen- United States Patent 2,702,859Patented Feb. 22, 1955 erated by a rolled throat feet horn 20, itselffed by a revolving or oscillating feed guide 22, asdisclosed anddescribed in patent application of Roy C. Spencer for Rolled Throat ScanHorn, executed October 15, 1945, Serial No. 628,283, filed November 13,1945. This feed horn 20 itself feeds a parabolic cylinder 24 for anauxiliary reflector. This creates an apparent moving line source 26 onthe opposite side of the parabolic cylinder 24 which moves normal toitself across the aperture of the zone plate reflector 28. The radiationfrom this is then used to feed a conical zone plate reflector 28. Thevarious zones or plates such as 29, 30, 31 are from a family of conesrelated to cone 28 as described above. This reflector 28 has its axis onthe source and of sufficient angle a so that the radiation 32 from itmisses the parabolic cylinder 24. This reduces the problems of matchingand frequency sensitivity which occur when radiation is reflected backinto a primary feed. Sending the reflected radiation 32 over a differentpath from the incident 34 also separates throat 36 and mouth 38 topermit rotary feed motion 22 at the throat of rolled feed horn 20. Sincethe zones are portions of cones they can be made of sheet metal, makingpossible a much simpler manufacturing process than the forming ofnon-developable antenna surfaces.

Other uses and adaptations of the elliptical cone reflectors arepossible and the principle of the invention should not be deemed limitedby the fact that specific embodiments are illustrated here.

What is claimed is:

1. In an antenna, a reflector characterized by the property ofreflecting a substantially plane wave front making an angle a with anilluminating line source, said reflector being a conical surface havingelliptical sections normal to said line source, each of said ellipticalsections having eccentricity equal to cos a and having a focus on theline source.

2. In an antenna, a reflector composed of a plurality of zones, eachzone being constructed as a portion of an elliptical cone as describedin claim 1, and each zone differing from the zone adjacent thereto byhaving the focal length thereof different by an amount of where N is anyinteger and A is the wave length in free space of the energy saidantenna is adapted to radiate.

3. In antenna, a reflector characterized by the property of reflecting asubstantially plane wave front making an angle a with an illuminatingline source, said reflector being a portion of a conical surface havingsections in parallel planes normal to said line source that are portionsof ellipses, each of said ellipses having a focus on said line sourceand a major axis disposed in a fixed plane including said line source,the vertices of said ellipses to one side of said line source beingcoincident with a straight line in said last-mentioned plane, saidstraight line making an angle of (1/2 with said line source, each ofsaid ellipses further having an eccentricity equal to cos a, angle abeing of a magnitude greater than 0 and less than 4. Apparatus as inclaim 3 wherein each of said elliptical sections is that portion of anellipse included between the vertex of the ellipse coincident with saidstraight line and the latus rectum of the ellipse closer to saidlastmentioned vertex.

5. In an antenna, a reflector composed of a plurality of zones, each ofsaid zones being constructed as a portion of a conical surface asdescribed in claim 3, wherein the focal length of the elliptical sectionof one zone differs from the focal length of the elliptical section ofan adjacent zone by an amount of NA/ (1+cos a), where N is an integerand )k is the wave length of the energy to be radiated in thepropagating medium included between said line source and said reflector,said focal lengths being measured along one predetermined linetransverse to said line source.

6. In an antenna, a reflector characterized by the property ofreflecting a substantially plane wave front in predeterminedrelationship to a fixed preselected line, said reflector being a conicalsurface having elliptical sections in planes normal to said line.

7. In an antenna, a reflector characterized by the property ofreflecting a substantially plane wave front in predeterminedrelationship to an illuminating line source, said reflector comprising aconical surface having elliptical sections in planes normal to said linesource.

8. In an antenna, a reflector characterized by the property ofreflecting a substantially plane wave front in predeterminedrelationship to an illuminating line source, said reflector comprising asurface that is a portion of an oblique elliptical cone and that haselliptical sections in planes normal to said line source.

9. A directional microwave reflector-type antenna for producing ascanning radiation beam comprising a line source of radiation, meansdirecting said line source in a reciprocating motion normal to said linesource thereby to produce a scan, a parabolic cylinder fed by said linesource in the region of the focus of said parabola thereby to reflectsaid radiation in a predetermined direction, and a reflector comprisinga plurality of zones fed by said parabolic cylinder, each zone beingconstructed as a portion of an elliptical cone as described in claim 1,and each zone diflering from the zone adjacent thereto by having thefocal length thereof different by an amount of NA/(1+cos a), where N isany integer and A is the wave length in free space of the energy saidantenna is adapted to radiate.

10. A directional microwave reflector-type antenna for producing ascanning radiation beam comprising a line source of radiation, meansdirecting said line source in a reciprocating motion normal to said linesource thereby to produce a scan, a first parabolic cylindricalreflector having a focus on said line source thereby to reflect saidradiation from said line source in a predetermined direction, and asecond reflector comprising a plurality of zones fed by said firstreflector, each of said zones being constructed as a portion of aconical surface as described in claim 6 wherein the focal length of theelliptical section of one zone differs from the focal length of theelliptical section of an adjacent zone by an amount NA/( 1+cos a), whereN is any integer and is the wave length of the energy to be radiated inthe propagating medium included between said line source from said firstreflector and said second reflector, said focal lengths being measuredalong one predetermined line transverse to said line source.

11. A directional microwave reflector-type antenna for producing ascanning radiation beam comprising, a first parabolic cylindricalsurface for reflecting a line source of radiation, a second reflectorcomposed of a plurality of zones fed by said line source from said firstreflector, each of said zones being constructed as a portion of aconical surface having sections in parallel planes normal to said sourcethat are portions of ellipses, wherein the focal length of theelliptical section of one zone differs from the focal length of anadjacent zone by an amount Nx/ l+cos a), where N is any integer and x isthe wave length of the energy to be radiated in the propagating mediumincluded bet-ween said line source and said second reflector, said focallengths being measured along one predetermined line transverse to saidline source, and an illuminating line source of radiation directed totraverse said first concave surface in a direction normal to said linesource thereby to produce a scan.

References Cited in the file of this patent UNITED STATES PATENTS796,254 Hartmann Aug. 1, 1905 1,735,377 Caughlan Nov. 12, 1923 1,906,546Darbord May 2, 1933 2,483,575 Cutler Oct. 4, 1949 FOREIGN PATENTS114,368 Austria Dec. 9, 1941

